Excimer or molecular fluorine laser having lengthened electrodes

ABSTRACT

An excimer or molecular fluorine laser system includes a laser tube filled with a gas mixture including fluorine and a buffer gas, and multiple electrodes within the laser tube connected with a pulsed discharge circuit for energizing the gas mixture. At least one of the electrodes is longer than 28 inches in length, preferably two main electrodes are each extended to greater than 28 inches in length. The laser system further includes a resonator including the laser tube for generating a pulsed laser beam having a desired energy. The laser system is configured such that an output beam would be emitted having an energy below the desired energy if each of the electrodes were 28 inches in length or less, and the laser system outputs a beam at the desired energy due to the length of the electrodes being extended to a length greater than 28 inches.

PRIORITY

[0001] This application claims the benefit of priority to U.S.provisional patent application No. 60/184,705, filed Feb. 24, 2000.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The invention relates to excimer and molecular fluorine lasers,and particularly for generating line-narrowed DUV and VUV laser beamsusing lasers with lengthened electrodes for compensating one or moreother factors tending to reduce the gain per resonator transit of thebeam.

[0004] 2. Discussion of the Related Art Line-narrowed and/orline-selected excimer and molecular fluorine lasers are advantageouslyused in industrial applications such as optical microlithography forforming small electronic structures on silicon substrates, photoablationand micromachining, among others. Such lasers generally include adischarge chamber containing two or more gases such as a halogen and oneor two rare gases. KrF (248 nm) and ArF (193 nm) excimer lasers areexamples of excimer lasers that are typically line-narrowed and have gasmixtures, respectively, of krypton, fluorine and a buffer gas typicallyof neon, and argon, fluorine and a buffer gas of neon and or helium (seeU.S. patent application Ser. Nos. 09/447,882, 09/734,459 and 09/513,025,09/602,184, 09/629,256, 09/640,595, 60/162,735, 60/178,445, 09/715,803,60/200,163, 09/584,420 and 60/215,933, which are assigned to the sameassignee as the present application and are hereby incorporated byreference). The molecular fluorine (F₂) laser has a gas mixture offluorine and one or more buffer gases, and emits at least two linesaround 157 nm, one of which may be selected, and narrowed, such that avery narrow linewidth VUV beam may be realized (see U.S. Pat. No.6,157,152 and U.S. patent application Ser. Nos. 09/317,695, 09/130,277,60/212,183, 09/482,698, 09/599,130, 60/173,993, 60/166,967, 60/657,396,09/317,527, 60/170,919 and 60/212,301, which are assigned to the sameassignee as the present application and are hereby incorporated byreference).

[0005] The KrF laser is perhaps the most commonly used laser forphotolithographic applications today. The ArF laser and the F₂ laserare, however, becoming and expected to become more prevalent forprocessing smaller structures due to their shorter wavelength emissionspectra. Each of the ArF and F₂ lasers exhibit lower gain and higherradiation losses, primarily due to optical absorption, than the KrFlaser.

RECOGNIZED IN THE INVENTION

[0006] It is recognized in the present invention that it is desired tohave an excimer or molecular fluorine laser system, particularly an ArFor F₂ laser system, having enhanced gain characteristics. It is alsorecognized in the present invention that is also desired to have anexcimer laser system exhibiting longer output emission pulses and/orlonger inversion times. Longer pulses and/or longer inversion times maybe achieved by using smaller pump intensities, or reduced electricalpower deposition per discharge volume, and/or reduced halogenconcentrations in the gas mixture. Such an excimer or molecular fluorinelaser would feature a greater number of round trips for the beam in theresonator. Among the other advantages of such a laser would be enhancedline narrowing by the line-narrowing unit typically included in theresonator setups of these lasers. Reducing the volumetric powerdeposition and/or reducing the fluorine concentration, however, wouldalso tend to cause the laser to exhibit reduced gain characteristics ifnot otherwise compensated.

SUMMARY OF THE INVENTION

[0007] It is an object of the invention to have an excimer or molecularfluorine laser having enhanced inherent gain characteristics.

[0008] It is another object of the invention to have an excimer ormolecular fluorine laser system that exhibits longer output pulsesand/or longer inversion times without sacrificing gain.

[0009] In accord with the above object, an excimer or molecular fluorinelaser is provided including a discharge tube filled with a gas mixture.An electrical discharge circuit is connected to multiple electrodeshoused within the discharge tube for energizing the gas mixture. A laserresonator including the discharge tube and preferably a line narrowingand/or selection unit for generating a line-narrowed and/orline-selected laser beam. At least one, and preferably both, of the mainelectrodes are significantly longer than conventional electrodes. Forexample, the main electrodes would be longer than 28 inches, andpreferably 30 to 40 inches long or greater, an even as long or longerthan 50 inches.

[0010] Advantageously, the lengthened main electrodes of the lasersystem provide enhanced gain. Particular application is for use with ArFand F₂ laser systems, which as described above, tend to exhibitcharacteristically lower gains than other excimer laser systems such asthe KrF laser.

[0011] Preferably, longer light pulses and/or longer inversion times areproduced by reducing the electrical power deposition per dischargevolume and/or by reducing the halogen concentration in the gas mixture.Also line-narrowing to 0.5 pm or less may be produced wherein loss ofgain is compensated by the lengthened electrodes. The gas pressurewithin the laser tube may have a reduced pressure than conventionallasers, while the lengthened electrodes compensate the decrease in laseroutput energy associated with the lower pressure in the tube. Thedischarge width may be reduced, e.g., by reducing the width of theelectrodes to enhance the clearing ratio of the gas mixture through thedischarge for operation at higher repetition rates, or the outputcoupler may have a reduced reflectivity, each of which is compensated bythe increased gain due to the lengthened electrodes. In each case, thereduced gain associated with this preferred feature is advantageouslycompensated by the lengthening of the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 schematically shows a block diagram of an excimer ormolecular fluorine laser system in accord with a preferred embodiment.

[0013]FIG. 2a schematically shows a side view of the laser chamber ofthe laser of FIG. 1.

[0014]FIG. 2b schematically shows a side view of the laser chamber ofthe laser of FIG. 1.

[0015]FIG. 3 schematically shows a cross-sectional view of the laserchamber of the laser of FIG. 1.

[0016]FIG. 4 schematically shows a cross-sectional view of a secondpreferred embodiment of the laser of FIG. 1.

[0017]FIG. 5 schematically shows a cross-sectional view of a thirdpreferred embodiment of the laser of FIG. 1.

[0018]FIG. 6 schematically shows a cross-sectional view of a fourthpreferred embodiment of the laser of FIG. 1.

[0019]FIG. 7 is a graph showing the dependence of the output THG poweron the crystal temperature.

INCORPORATION BY REFERENCE

[0020] Several references are cited herein which are, in addition tothose references cited above and below herein, including that which isdescribed as background, and the above invention summary, are herebyincorporated by reference into the detailed description of the preferredembodiments below, as disclosing alternative embodiments of elements orfeatures of the preferred embodiments not otherwise set forth in detailbelow. A single one or a combination of two or more of these referencesmay be consulted to obtain a variation of the preferred embodimentsdescribed in the detailed description below. Further patent, patentapplication and non-patent references are cited in the writtendescription and are also incorporated by reference into the detaileddescription of the preferred embodiment with the same effect as justdescribed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021]FIG. 1 shows a schematic block diagram of a laser system in accordwith a preferred embodiment. FIG. 1 shows various modules of an excimeror molecular fluorine laser for deep ultraviolet (DUV) or vacuumultraviolet (VUV) lithography, while the laser system (not includingmodules 26 and 28) may be used for other industrial applications, asmentioned above. It is recognized herein that it is desired to have alaser pulse with an extended duration over conventional systems and inaccord with the lifetime of the illumination and discharge andprojection optical system. The preferred embodiments may be usedadvantageously to provide a stabilized long pulse emission. Otheradvantageous modifications of conventional excimer or molecular fluorinelaser systems may be made wherein gain may be lost, while the lengthenedelectrodes of the preferred embodiments advantageously compensate thoselosses.

[0022] Another specific field for the application of the presentinvention is the manufacturing of flat panel displays by TFT annealing.The TFT annealing process is strongly nonlinear. It is thereforerecognized that stabilization of the temporal pulse shape and thereforepeak intensity would be advantageous to produce reliable manufacturingresults of the annealing process. Micromachining, photoablation andother industrial application may also be performed advantageously with asimilar laser system.

[0023] The system shown generally includes a laser chamber or dischargetube 2 having a pair of main electrodes 3 therein connected with asolid-state pulser module 4. A gas handling and exchange module 6 isshown connected to the laser chamber 2. The solid-state pulser module 4is powered by a high voltage power supply 8. The laser chamber 2 issurrounded by optics module 10 and optics module 12, forming aresonator. The optics modules 10 and 12 are controlled by an opticscontrol module 14, or alternatively directly controlled by the processor16.

[0024] The processor or computer 16 for laser control receives variousinputs and controls various operating parameters of the system. Adiagnostic module 18 receives and measures various parameters of a splitoff portion of the main beam 20 via a beam splitter 22. The beam 20 isthe laser output to an imaging system (not shown). The laser controlcomputer 16 communicates through an interface 24 with a stepper/scannercomputer 26 and other control units 28.

[0025] The laser chamber 2 contains a laser gas mixture and includes apair of main discharge electrodes and one or more preionizationelectrodes (not shown). The electrodes 3 are described in more detailbelow with reference to FIGS. 2 and 3.

[0026] The laser resonator which surrounds the laser chamber 3containing the laser gas mixture includes optics module 10 includingline-narrowing and/or line-selection optics for a line narrowed excimeror molecular fluorine laser, which may be replaced by a highreflectivity mirror or the like if line-narrowing is not desired.Exemplary line-narrowing optics of the optics module 10 include a beamexpander including multiple beam expanding prisms, an optional etalonand a diffraction grating, which produces a relatively high degree ofdispersion, for a narrow band laser such as is used with a refractive orcatadioptric optical lithography imaging system. For a semi-narrow bandlaser such as is used with an all-reflective imaging system, the gratingmay be replaced with a highly reflective mirror, and a lower degree ofdispersion may be produced by a dispersive prism.

[0027] The beam expander of the line-narrowing optics of the opticsmodule 10 typically includes one or more prisms. The beam expander mayinclude other beam expanding optics such as a lens assembly, reflectiveoptics or a converging/diverging lens pair. The grating or highlyreflective mirror is preferably rotatable so that the wavelengthsreflected into the acceptance angle of the resonator can be selected ortuned. The grating is typically used, particularly in KrF and ArFlasers, both for achieving narrow bandwidths and also often forretroreflecting the beam back toward the laser tube. One or moredispersive prisms may also be used, and more than one etalon may beused. An output coupling interferometer or coupled resonator may be used(see the '803 application, incorporated by reference above and herein).

[0028] Depending on the type and extent of line-narrowing and/orselection and tuning that is desired, and the particular laser that theline-narrowing optics of the optics module 10 is to be installed into,there are many alternative optical configurations that may be used. Forthis purpose, those shown in U.S. Pat. Nos. 4,399,540, 4,905,243,5,226,050, 5,559,816, 5,659,419, 5,663,973, 5,761,236, and 5,946,337,and U.S. patent application Ser. Nos. 09/317,695, 09/130,277,09/244,554, 09/317,527, 09/073,070, 09/452,353, 09/598,552, 09/629,256,09/599,130, and any of the patents or patent applications set forthabove in the background, each of which is assigned to the same assigneeas the present application, and U.S. Pat. Nos. 5,095,492, 5,684,822,5,835,520, 5,852,627, 5,856,991, 5,898,725, 5,901,163, 5,917,849,5,970,082, 5,404,366, 4,975,919, 5,142,543, 5,596,596, 5,802,094,4,856,018, 5,970,082, 5,978,409, 6,081,542, 6,061,382, 5,999,318,5,150,370 and 4,829,536, are each hereby incorporated by reference intothe present application.

[0029] Optics module 12 preferably includes means for outcoupling thebeam 20, such as a partially reflective resonator reflector or outputcoupling interferometer. The beam 20 may be otherwise outcoupled such asby an intra-resonator beam splitter or partially reflecting surface ofanother optical element, and the optics module 12 would in this caseinclude a highly reflective mirror.

[0030] The laser chamber 3 is sealed by windows transparent to thewavelengths of the emitted laser radiation 14. The windows may beBrewster windows or may be aligned at another angle to the optical pathof the resonating beam.

[0031] After a portion of the output beam 20 passes the outcoupler ofthe optics module 12, that output portion impinges upon the beamsplitter 22 (or beam splitter module including means for separating theVUV emission from visible emission, as described in the '552, '952 or'967 applications, mentioned above) which reflects a portion of the beamto the diagnostic module 18. The portion of the outcoupled beam whichtraverses the beam splitter 22 is the output beam 20 of the laser, whichpropagates toward an industrial or experimental application such as animaging system for photolithographic applications. The optics controlmodule 14 controls the optics modules 10 and 12 such as by receiving andinterpreting signals from the processor 16, and initiating realignmentor reconfiguration procedures (see the '353, '695, 277, '554, and '527applications mentioned above).

[0032] The solid-state pulser module 14 and high voltage power supply 8supply electrical energy in compressed electrical pulses to thepreionization and main electrodes 3 within the laser chamber 2 toenergize the gas mixture. The preferred pulser module and high voltagepower supply are described at U.S. Pat. Nos. 6,020,723 and 6,005,880 andU.S. patent application Ser. Nos. 60/204,095, 60/223,027 and 09/390,146,which are assigned to the same assignee as the present application andare hereby incorporated by reference into the present application. Otherpulser modules are described at U.S. Pat. Nos. 5,982,800, 5,982,795,5,940,421, 5,914,974, 5,949,806, 5,936,988, 6,028,872 and 5,729,562,each of which is hereby incorporated by reference. A conventional pulsermodule may generate electrical pulses in excess of 3 Joules ofelectrical power (see the '988 patent, mentioned above). Electricalpulses of reduced electrical power may be generated in preferredembodiments, while still achieving laser pulses of a same desiredenergy, as will be understood from the discussion below with respect tothe lengthened electrodes of FIGS. 2 and 3.

[0033] The diagnostic module 18 includes at least one energy detector.This detector measures an energy of the beam portion that correspondsdirectly to the energy of the output beam 20. An optical attenuator suchas a plate or a coating may be formed on or near the detector to controlthe intensity of the radiation impinging upon the detector (see U.S.patent application Ser. Nos. 09/172,805 and 60/178,620, each of which isassigned to the same assignee as the present application and is herebyincorporated by reference).

[0034] The same or a different detector may be used to measure the timeresolved pulse intensity, or pulse shape, of the beam portion (see the'818 application). If two detectors are used for these two measurements,then a beam splitter may be used to direct beam portions to therespective detectors.

[0035] A portion of the beam is also preferably directed to a wavelengthand bandwidth detection module, again preferably using a beamsplitter.The module preferably includes a monitor etalon such as is described atU.S. patent application Ser. No. 09/416,344 and U.S. Pat. No. 4,905,243,each of which is assigned to the same assignee as the presentapplication, and U.S. Pat. No. 5,450,207, all of which are herebyincorporated by reference. The wavelength and bandwidth detection modulemonitors the wavelength and bandwidth, and sends the wavelength andbandwidth information to the processor 16 and/or directly to the opticscontrol module 14. The wavelength and bandwidth may be adjusted based onthe information the processor 16 and/or optics control module 14receives from the diagnostic module 18 based on its programming and thedesired wavelength and/or bandwidth for the particular industrialapplication being performed.

[0036] The processor or control computer 16 receives and processesvalues of the pulse shape, amplified spontaneous emission, energy,energy stability, wavelength, and/or bandwidth of the output beam andcontrols the line narrowing module to tune the wavelength and/orbandwidth, and controls the power supply and pulser module 4 and 8 tocontrol the energy. In addition, the computer 16 controls the gashandling module 6 which includes gas supply valves connected to variousgas sources.

[0037] The laser gas mixture is initially filled into the laser chamber2 during new fills. The gas composition for a very stable excimer laserin accord with the preferred embodiment uses helium or neon or a mixtureof helium and neon as buffer gas, depending on the laser. Preferred gascompositions are described at U.S. Pat. Nos. 4,393,405, 6,157,162 and4,977,573 and U.S. patent application Ser. Nos. 09/447,882, 09/418,052,09/688,561 and 09/513,025, each of which is assigned to the sameassignee and is hereby incorporated by reference into the presentapplication. The concentration of the fluorine in the gas mixture mayrange from 0.003% to 1.00%, and is preferably around 0.1%. For the KrFand ArF lasers, the concentration of the krypton and argon,respectively, is around 1%. An additional gas additive, preferably arare gas such as xenon, may be added for increased energy stabilityand/or as an attenuator as described in the '025 application, mentionedabove.

[0038] For the F₂-laser, an addition of Xenon and/or Argon may be used.The concentration of xenon or argon in the mixture may range from0.0001% to 0.1%. For the ArF-laser, an addition of xenon or krypton maybe used also having a concentration between 0.0001% to 0.1%. For theKrF-laser, an additive of xenon or argon may be used having aconcentration again in a range from 0.0001% to 0.1%.

[0039] Halogen and rare gas injections, total pressure adjustments andgas replacement procedures are performed using the gas handling module 6preferably including a vacuum pump, a valve network and one or more gascompartments. The gas handling module 6 receives gas via gas linesconnected to gas containers, tanks, canisters and/or bottles. Preferredgas compositions of the various excimer lasers and the molecularfluorine laser and preferred gas handling and/or replenishmentprocedures of the present invention, other than specifically describedherein, are described at U.S. Pat. Nos. 4,977,573 and 5,396,514 and U.S.patent application Ser. Nos. 09/447,882, 09/418,052, 09/379,034, and09/688,561, each of which is assigned to the same assignee as thepresent application, and U.S. Pat. Nos. 5,978,406, 6,014,398 and6,028,880, all of which are hereby incorporated by reference. A xenongas supply is also preferably included either internal or external tothe laser system according to the '025 application, mentioned above.

[0040]FIG. 2 schematically shows a side view of the laser chamber 3 ofthe excimer or molecular fluorine laser of FIG. 1. A pair of mainelectrodes 30 and 32 separated by a discharge area 34, and one or morepreionization units 36, are housed within the laser chamber 2. Thepreionization units 36 are shown held in position by a number ofsupports 38. The number of these supports 38 depends on the size andcomposition of the preionization units 36, which may be increased overconventional systems in accord with the preferred embodiment (seebelow). The supports 38 are preferably made of a dielectric material(for example ceramics) to avoid disturbances of the electrical field bythe supports 38. Exemplary preionization units 36 are described in U.S.patent application Ser. Nos. 09/247,887, 09/532,276 and 09/587,996 and09/692,265, each of which is assigned to the same assignee as thepresent application, and at U.S. Pat. Nos. 5,337,330 and 5,719,896, allof which are hereby incorporated by reference.

[0041] The chamber 2 preferably comprises a dielectric frame 40 whichinsulates the high voltage main electrode 30 which is connected to thehigh voltage power supply 8 and solid-state pulser module 4, discussedabove. One or more feedthroughs 42 are shown sealably penetrating thelaser chamber 2 from the outside, and advantageously allowing external(i.e., to the discharge chamber) circuitry 44 (not shown, but see the'723 patent, mentioned above) to be connected to the preionization units36.

[0042] Exemplary cross-sectional configurations for the main electrodes30 and 32 are described at U.S. patent application Ser. Nos. 09/453,670and 09/692,265, and U.S. Pat. Nos. 4,860,300, 5,347,532 and 5,729,565,and German Patent No. DE 44 01 892, each of which is assigned to thesame assignee as the present application and which is herebyincorporated by reference. The preferred cross-sectional configurationsare discussed in more detail below with reference to FIG. 3.

[0043] The main electrodes 30 are significantly longer than conventionalelectrodes. That is, the main electrodes 30, 32 are longer than 28inches (see U.S. Pat. No. 5,856,991). Preferably, the main electrodes30, 32 are 30 to 40 inches long, and may be still longer depending onthe application and configuration of other laser system components. Thelengthened electrodes 30, 32 may be assembled into an otherwiseconventional laser chamber, or the laser chamber 3 may be extended toaccommodate the lengthened electrodes 30, 32. The preionization units 36may also be lengthened in proportion with the lengthening of the mainelectrodes 30, 32. Particular embodiments of laser systems havinglengthened electrodes, i.e., longer than 28 inches and up to 40 inchesor more, and other modifications from conventional laser systems aredescribed below.

[0044] The volume of gas in the discharge area 34 between the electrodes30, 32 is advantageously increased in the preferred embodiment due tothe greater length of the electrodes 30, 32. Advantageously, thisgreater discharge volume is achieved without increasing the dischargewidth, which can otherwise increase demands on the gas flow vessel andpulse power module. Greater numbers of spontaneous and stimulatedemission photons are generated as a result of this discharge volumeincrease, and thus greater laser gain is realized in the laser chamber2. The enhanced gain feature of the preferred embodiment is particularlyadvantageous for laser systems that exhibit characteristically lowgains, such as the ArF and F₂ lasers. For these lasers, the lengtheningof the electrodes may be performed as the sole modification of anotherwise conventional ArF or F₂ laser system to advantageously increasethe gain. The enhanced gain is further advantageous for other laserssuch as the KrF laser, and particularly for compensating furthermodifications. The enhanced gain feature is particularly advantageousfor line-narrowed lithography lasers, wherein all but a very narrowspectral band (e.g., <0.5 pm) is filtered out from the characteristicemission of the laser and output pulse energy and energy dose demandsremain high.

[0045] Other embodiments are also enabled using the feature of thelengthened electrodes 30, 32 of the present invention. For example,advantageously longer laser pulses and/or longer inversion times may beachieved in an excimer or molecular fluorine laser by using smaller pumpintensities (having reduced peak power), or reduced electrical powerdeposition per discharge volume. Reducing the halogen concentration inthe gas mixture can also achieve longer laser pulses and/or longerinversion times. Reducing the pump intensity and reducing the halogenconcentration in combination will produce still longer laser pulsesand/or still longer inversion times.

[0046] An excimer or molecular fluorine laser exhibiting longer pulsesand/or inversion times would feature a beam that undergoes a greaternumber of round trips in the laser resonator. Among the other advantagesof such a laser would be enhanced line narrowing. As discussed above,reducing the volumetric power deposition and/or reducing the fluorineconcentration to achieve the longer pulses or inversion times would tendto cause a conventional laser to exhibit reduced gain characteristics.The enhanced gain achieved by using the lengthened electrodes 30, 32 ofthe preferred embodiment, however, advantageously compensates anyreduction in gain caused by reducing the volumetric power depositionand/or reducing the fluorine concentration in the gas mixture.

[0047]FIG. 3 schematically shows a cross-sectional view of the laserchamber 2 of the laser of FIGS. 1 and 2. Two preionizer units 36 areshown, and a single preionizer may be used, as are the main electrodes30, 32 separated by the discharge area 34. Preferred cross-sectionalshapes of the main electrodes 30 and 32 are described at the '670application, mentioned above.

[0048] A set of peaking capacitors Cp is also shown which are connectedto the high voltage electrode 30. The peaking capacitors Cp are used forelectrical pumping of the main discharge. A set of sustaining capacitorsmay also be included (see the '595 application, mentioned above). Thepreionization units 36 are preferably connected to external circuitry 44as discussed above with reference to the '265 application, and may bealternatively connected to the ground electrode.

[0049]FIG. 3 also shows one or more dielectric insulators 40, or adielectric frame. The dielectric insulators 40 may be straight, asshown, or curved, e.g., to provide a more aerodynamic electrode chamber.The insulators 40 may also be straight, but tilted such as to form atrapezoidally shaped electrode chamber (see the '670 application,mentioned above). A pair of preferred spoilers 46, preferably integratedwith the insulators 40 or insulating frame, are shown (again, see the'670 application, mentioned above). The spoilers 46 provide a moreuniform gas flow from the reservoir or gas flow vessel 48 through thedischarge area and are preferably positioned to shield the preionizationunits 36 from the main electrode 32, as shown.

[0050] Several embodiments will now be described that are enabledparticularly by the feature of all of those embodiments of havinglengthened electrodes over conventional systems. In each embodiment, theexcimer or molecular fluorine laser system may be the same orsubstantially similar to the laser system set forth above, or may differin one aspect having advantages that would tend to reduce the lasergain, except that the gain is compensated by the lengthened electrodes.

[0051] A first embodiment includes an excimer or molecular fluorinelaser system having a discharge chamber filled with a gas mixture oftypical composition. Main and preionization electrodes are housed withinthe discharge chamber and connected to a pulsed power supply circuit.The discharge chamber is within a laser resonator includingline-narrowing optics for producing a line-narrowed output beam. Thepulsed power supply circuit is configured to supply electrical pulses tothe electrodes, preferably according to signals received from aprocessor that is programmed with voltage tables and is monitoring anoutput energy of the beam by receiving signals from an energy detector.The electrical pulses that the power supply circuit supplies areinsufficient to produce output laser pulses of a desired energy (e.g.,around 10 mJ for lithographic processing), if the electrodes were 28inches in length or a conventional length.

[0052] For example, an electrical pulse corresponding to a voltage ofbetween 770 and 790 Volts loaded on a main storage capacitor of thedischarge circuit prior to opening of a solid state switch to deliverthe charge to the electrodes, may be applied to produce laser pulses atthe desired energy for electrodes of 28 inches in length, while thesystem of the first embodiment may apply an electrical pulsecorresponding to less than 770 Volts, and preferably less than 750Volts, and more preferably less than 700 Volts, or less than 90% of thatloaded for the system having 28 inch or conventional electrodes, loadedon the main storage capacitor and applied through the switch to thelengthened electrodes to produce the desired output energy. Theelectrodes are longer than 28 inches or conventional length and are alength sufficient to produce the output pulses at the desired energywhen the electrical pulses, corresponding to reduced voltage loaded ontothe main storage capacitor, are applied to them.

[0053] Longer light pulses and/or longer inversion times are produced byreducing the electrical power deposition per discharge volume. Thisproduces enhanced energy stability. Additionally, the reduced electricalpower applied per pulse provides greater discharge circuit componentlifetimes, reduced voltage load on the solid state switch and anenhanced ability to increase the applied voltage when the gas mixture,laser tube and/or optics age to maintain the desired output beam energy.

[0054] A second embodiment includes an excimer or molecular fluorinelaser system having a discharge chamber filled with a gas mixture of acomposition including a reduced percentage of fluorine, e.g., less than0.1% and possibly less than 0.08% or even 0.06%. Main and preionizationelectrodes are housed within the discharge chamber and connected to apulsed power supply circuit. The discharge chamber is within a laserresonator including line-narrowing optics for producing a line-narrowedoutput beam. The pulsed power supply circuit is configured to supplyelectrical pulses to the electrodes, preferably according to signalsreceived from a processor that is programmed with voltage tables and ismonitoring an output energy of the beam by receiving signals from anenergy detector. The electrical pulses that the power supply circuitsupplies are sufficient to produce output laser pulses of a desiredenergy (e.g., around 10 mJ for lithographic processing), for systemshaving electrodes that are 28 inches in length or a conventional length.The reduction in fluorine concentration, however, causes the energy ofthe output beam to be reduced from the desired energy. The electrodesare, however, longer than 28 inches and are a length sufficient toproduce the output pulses at the desired energy when the electricalpulses are applied to them across the gas mixture in the dischargeregion having the reduced fluorine concentration from a conventionalcomposition.

[0055] The reduction in fluorine concentration provides a narrowing ofthe bandwidth of the output laser pulses (see U.S. patent applicationSer. No. 5,835,520, which is hereby incorporated by reference). In thisway, a laser resonator having line-narrowing components sufficient tonarrow the bandwidth to around 0.5 pm, e.g., using a multiple prism beamexpander and a grating such as for a conventional ArF laser, might benarrowed to substantially less than 0.5 pm due to the reduced fluorineconcentration while the pulse energy would be at the desired energy.Even an improved ArF laser employing an outcoupling interferometer orcoupled resonator such as is described in the '803 application wouldhave a still narrower bandwidth. Otherwise conventional KrF and F₂lasers would benefit similarly by reducing the fluorine concentrationand correspondingly the bandwidth, and lengthening the electrodes tolonger than 28 inches or their conventional lengths.

[0056] A third embodiment includes an excimer or molecular fluorinelaser system having a discharge chamber filled with a gas mixture of acomposition including an ordinary concentration of fluorine, e.g.,around 0.1%, and including a trace amount of a gas additive such as maybe described in the application Ser. No. 09/513,025, mentioned above.The gas additive serves to increase the energy stability and burstovershoot control, and at least in sufficient amounts of the traceadditive, the energy of the output pulses is reduced. Main andpreionization electrodes are housed within the discharge chamber andconnected to a pulsed power supply circuit. The discharge chamber iswithin a laser resonator including line-narrowing optics for producing aline-narrowed output beam. The pulsed power supply circuit is configuredto supply electrical pulses to the electrodes, preferably according tosignals received from a processor that is programmed with voltage tablesand is monitoring an output energy of the beam by receiving signals froman energy detector. The electrical pulses that the power supply circuitsupplies are sufficient to produce output laser pulses of a desiredenergy (e.g., around 10 mJ for lithographic processing), for systemshaving electrodes that are 28 inches in length or a conventional length.The addition of the trace gas additive, e.g., of xenon, in the gasmixture, however, causes the energy of the output beam to be reducedfrom the desired energy. The electrodes are, however, longer than 28inches and are a length sufficient to produce the output pulses at thedesired energy when the electrical pulses are applied to them across thegas mixture in the discharge region having the trace gas additive.

[0057] The gas additive may even be added in a greater amount to furtherimprove the energy stability or burst overshoot control over thatdescribed in the '025 application, while the lengthening of theelectrodes according to the preferred embodiment compensates thereduction in energy. Thresholds of 12 ppm, 17 ppm, 30 ppm and 100 ppmare discussed in the '025 application, as are alternate gas additives toxenon or another noble gas (see also U.S. Pat. No. 6,151,350, which ishereby incorporated by reference). As with any of these embodiments, thedriving voltage may also be adjusted to achieve an output beam of thedesired energy, and as in the case of the first embodiment above, theapplied high voltage may even be reduced.

[0058] A fourth embodiment includes an excimer or molecular fluorinelaser system having a discharge chamber filled with a gas mixture of anordinary composition, e.g., including an ordinary concentration offluorine, e.g., around 0.1%, and possibly including a trace amount of agas additive such as may be described in the 09/513,025 application,mentioned above, wherein the system is preferably compensated in thisembodiment, or any other embodiments having the trace additive but thethird embodiment, as in the '025 application for any affect the gasadditive may have on the output beam energy. Main and preionizationelectrodes are housed within the discharge chamber and connected to apulsed power supply circuit. The discharge chamber is within a laserresonator including line-narrowing optics for producing a line-narrowedoutput beam.

[0059] In this fourth embodiment, the line-narrowing includes additionalline-narrowing optical elements or same elements configured for greaterline-narrowing than in conventional systems. For example, an extra beamexpanding or dispersion prism may be added, the dispersion of thegrating or prism may be increased, an aperture size may be reduced, ablaze angle of a grating may be increased (see the '256 application,mentioned above), an etalon or other interferometric device may beadded, and/or a free spectral range of the etalon or otherinterferometric device may be adjusted, among other techniques known tothose skilled in the art, each to reduce the bandwdith, e.g., to 0.5 pmor less from a wider bandwidth above 0.5 pm, or to 0.4 pm or less, oreven 0.3 pm or less, and generally to a bandwidth that as a result ofthe reduction, the energy of the beam is reduced to below a desiredenergy. That is, the modified configuration for producing the enhancedline-narrowing of the laser resonator would typically result in reducedoutput beam energy, all else being equal. However, the lengthening ofthe main electrodes serves compensate that loss of energy. The pulsedpower supply circuit is configured to supply electrical pulses to theelectrodes, preferably according to signals received from a processorthat is programmed with voltage tables and is monitoring an outputenergy of the beam by receiving signals from an energy detector. Theelectrical pulses that the power supply circuit supplies are sufficientto produce output laser pulses of a desired energy (e.g., around 10 mJfor lithographic processing), for systems having electrodes that are 28inches in length or a conventional length.

[0060] Thus, reduced bandwidth is achieved which is desired particularlyfor use with catadioptric photolithography imaging systems, while lossof energy is compensated by the lengthening of the electrodes. For theF₂-laser, e.g., a grating might not be chosen to be used because of itsreduced performance at 157 nm such that the laser output energy would betoo low. However, the reduction in energy may be compensated bylengthening the electrodes according to this fourth embodiment, so thatthe grating could be used and the linewidth of the output beam of the F₂laser advantageously reduced.

[0061] A fifth embodiment includes an excimer or molecular fluorinelaser system having a discharge chamber filled with a gas mixture of anordinary composition, e.g., including an ordinary concentration offluorine, e.g., around 0.1%. Main and preionization electrodes arehoused within the discharge chamber and connected to a pulsed powersupply circuit. The discharge chamber is within a laser resonatorincluding line-narrowing optics for producing a line-narrowed outputbeam. The pulsed power supply circuit is configured to supply electricalpulses to the electrodes, preferably according to signals received froma processor that is programmed with voltage tables and is monitoring anoutput energy of the beam by receiving signals from an energy detector.The electrical pulses that the power supply circuit supplies aresufficient to produce output laser pulses of a desired energy (e.g.,around 10 mJ for lithographic processing), for systems having electrodesthat are 28 inches in length or a conventional length.

[0062] The electrodes are configured in this fifth embodiment such thatthe discharge has a reduced width. For example, the discharge width maybe reduced to an effective line-narrowed resonator aperture width ofless than 3 mm or 4 mm, or may be reduced to 2 mm or less, as ispreferred, and even as low as 1 mm or less, depending on the materialsand ability of the electrodes to avoid wear. The discharge widthpreferably depends on the geometry of a raised central portion of theelectrodes 30, 32 according to the preferred embodiment shown in FIG. 3,and can depend on the overall width of the electrodes 3 in otherembodiments. The reduced discharge width allows the clearing ratio ofthe laser to be improved, which is the ability of the gas mixture withinthe discharge volume during one discharge to fully clear the dischargevolume making way for fresh gas mixture to fill the discharge volume forthe next discharge. Since the clearing ratio depends on the dischargewidth divided by the gas flow speed through the discharge, this fifthembodiment is advantageous. It is desired in the art to have excimer andmolecular fluorine lasers operating at higher repetition rates such as1-2 kHz and higher. As the repetition rate increases, the clearing ratioalso increases, and reducing the discharge width produces an increase inthe clearing ratio.

[0063] Reducing the discharge width would tend to reduce the energy ofthe beam, all else being equal. However, advantageously according thethe preferred embodiment, the electrodes are lengthened to compensatethe reduction in width. As an illustration, the output energy will tendto depend on the discharge volume, which is the cross sectional area(see FIG. 3) multiplied by the length of the electrodes, for an idealhomogeneous discharge. If the discharge width is reduced by 20% and theelectrodes lengthened by 20%, then the discharge volume is the same asbefore, and the advantage of reduced discharge width are realizedwithout loss of laser output energy.

[0064] Sixth and seventh embodiments includes the laser system designgenerally as understood from the above discussion and FIGS. 1-3. In thesixth embodiment, the reflectivity of the output coupler is reduced andthe electrodes lengthened to compensate the reduction in energyassociated with the reduction in outcoupler reflectivity. Thereflectivity may be reduced to substantially less than 22.5% and may beless than 20% or 15%, or even 10%. Reducing the reflectivity of theoutcoupler may be advantageous for setting a preferred temporal pulseshape, and depending on a balancing of gain with an inversion time, areduction in reflectivity may produce improved performance for somelaser systems.

[0065] In the seventh embodiment, the pressure in the laser tube isreduced to improve laser performance and the electrodes lengthened tocompensate the reduction in energy associated with the reduction inlaser tube pressure. The total pressure of the gas mixture in the lasertube may be reduced to below 3.0 bar, or below 2.5 bar or even below 2.0bar, e.g., such that the laser output beam energy would be reduced bythe reduction of the total pressure from that when a higher, moretypical value of the total pressure is used. Advantageously, the longerelectrodes of the preferred embodiment compensate this reduced energydue to the reduced total gas mixture pressure.

[0066] This above preferred embodiments meet the above objects of theinvention. An excimer or molecular fluorine laser has been describedthat exhibits enhanced gain characteristics resulting from thelengthening of the electrodes. An advantageous excimer or molecularfluorine laser system has also been described that exhibits longeroutput pulses and/or longer inversion times without sacrificing gain.

[0067] Those skilled in the art will appreciate that the just-disclosedpreferred embodiments are subject to numerous adaptations andmodifications without departing from the scope and spirit of theinvention. Therefore, it is to be understood that, within the scope andspirit of the invention, the invention may be practiced other than asspecifically described above.

What is claimed is:
 1. An excimer or molecular fluorine laser system,comprising: a laser tube filled with a gas mixture including fluorineand a buffer gas; a plurality of electrodes within the laser tubeconnected with a pulsed discharge circuit for energizing the gasmixture, at least one of said electrodes being longer than 28 inches inlength; a resonator including the laser tube for generating a pulsedlaser beam having a desired energy, wherein the laser system isconfigured such that an output beam would be emitted having an energybelow the desired energy if each of the electrodes were 28 inches inlength or less, and wherein the laser system outputs a beam at thedesired energy due to the length of said at least one of said electrodesbeing extended to a length greater than 28 inches.
 2. The laser systemof claim 1, wherein said at least one of said electrodes is longer than30 inches in length.
 3. The laser system of claim 1, wherein said atleast one of said electrodes is longer than 35 inches in length.
 4. Thelaser system of claim 1, wherein said at least one of said electrodes islonger than 40 inches in length.
 5. The laser system of claim 1, whereinsaid at least one of said electrodes is longer than 50 inches in length.6. The laser system of any of claims 2-4, wherein an input drivingvoltage is below that which would be applied to achieve the desiredoutput beam energy for a laser system having electrodes of 28 inches inlength.
 7. The laser system of claim 6, wherein said discharge circuitincludes a high voltage power supply, a main storage capacitor and asolid state switch, and charge loaded onto the capacitor by the highvoltage power supply is discharged through the switch to applyelectrical pulses to the electrodes during laser operation, and whereina charge loaded on said main storage capacitor to produce the electricalpulses is insufficient to produce the desired output beam energy for alaser system having electrodes of 28 inches in length, and does produceelectrical pulses sufficient to produce the desired output beam energyfor said laser system having electrodes longer than 28 inches in length.8. The laser system of claim 7, wherein the charge loaded on said mainstorage capacitor to produce the electrical pulses is below 770 Volts.9. The laser system of claim 7, wherein the charge loaded on said mainstorage capacitor to produce the electrical pulses is below 750 Volts.10. The laser system of claim 7, wherein the charge loaded on said mainstorage capacitor to produce the electrical pulses is below 700 Volts.11. The laser system of any of claims 2-4, wherein a fluorineconcentration in the gas mixture is less than 0.095%.
 12. The lasersystem of any of claims 2-4, wherein a fluorine concentration in the gasmixture is less than 0.08%.
 13. The laser system of any of claims 2-4,wherein a fluorine concentration in the gas mixture is less than 0.07%.14. The laser system of any of claims 2-4, wherein said gas mixturefurther includes more than 12 ppm of a gas additive for increasingenergy stability and burst overshoot control, while decreasing outputbeam energy.
 15. The laser system of any of claims 2-4, wherein said gasmixture further includes more than 17 ppm of a gas additive forincreasing energy stability and burst overshoot control, whiledecreasing output beam energy.
 16. The laser system of any of claims2-4, wherein said gas mixture further includes more than 30 ppm of a gasadditive for increasing energy stability and burst overshoot control,while decreasing output beam energy.
 17. The laser system of any ofclaims 2-4, wherein said gas mixture further includes more than 100 ppmof a gas additive for increasing energy stability and burst overshootcontrol, while decreasing output beam energy.
 18. The laser system ofany of claims 2-4, wherein the resonator includes line-narrowing opticsfor reducing a bandwidth of the output beam to 0.5 pm or less, while thedesired output beam energy would be produced with line-narrowing to abandwidth of greater than 0.5 pm.
 19. The laser system of any of claims2-4, wherein the resonator includes line-narrowing optics for reducing abandwidth of the output beam to 0.4 pm or less, while the desired outputbeam energy would be produced with line-narrowing to a bandwidth ofgreater than 0.4 pm.
 20. The laser system of any of claims 2-4, whereinthe resonator includes line-narrowing optics for reducing a bandwidth ofthe output beam to 0.3 pm or less, while the desired output beam energywould be produced with line-narrowing to a bandwidth of greater than 0.3pm.
 21. The laser system of any of claims 2-4, wherein a discharge widthis substantially 4 mm or less, while the desired output beam energywould be produced with a discharge width of greater than substantially 4mm.
 22. The laser system of any of claims 2-4, wherein a discharge widthis substantially 2 mm or less, while the desired output beam energywould be produced with a discharge width of greater than substantially 2mm.
 23. The laser system of any of claims 2-4, wherein a discharge widthis substantially 1 mm or less, while the desired output beam energywould be produced with a discharge width of greater than substantially 1mm.
 24. The laser system of any of claims 2-4, wherein a total pressureof the gas mixture within the laser tube is less than 3.0 bar, while thedesired output beam energy would be produced with a total pressure ofgreater than or equal to 3.0 bar.
 25. The laser system of any of claims2-4, wherein a total pressure of the gas mixture within the laser tubeis less than 2.5 bar, while the desired output beam energy would beproduced with a total pressure of greater than or equal to 3.0 bar. 26.The laser system of any of claims 2-4, wherein a total pressure of thegas mixture within the laser tube is less than 2.0 bar, while thedesired output beam energy would be produced with a total pressure ofgreater than or equal to 3.0 bar.
 27. The laser system of any of claims2-4, wherein a reflectivity of an output coupler of the laser resonator,is less than 22.5%, while the desired output beam energy would beproduced with a reflectivity of greater than or equal to 22.5%.
 28. Thelaser system of any of claims 2-4, wherein a reflectivity of an outputcoupler of the laser resonator, is less than 20%, while the desiredoutput beam energy would be produced with a reflectivity of greater thanor equal to 20%.
 29. The laser system of any of claims 2-4, wherein areflectivity of an output coupler of the laser resonator, is less than15%, while the desired output beam energy would be produced with areflectivity of greater than or equal to 15%.
 30. The laser system ofclaim 1, wherein the gas mixture further includes argon, and the lasersystem is an argon fluoride laser system.
 31. The laser system of claim1, wherein the buffer gas has a composition of around 99.9%, and thelaser system is a molecular fluorine laser system.
 32. The laser systemof claim 1, wherein the gas mixture further includes krypton, and thelaser system is a krypton fluoride laser system.